The present invention relates to plastic coated structures generally and more specifically to heat exchangers for use in a corrosive environment and to the method of manufacture thereof.
In the copending application of Irving D. Press, Ser. No. 773,750, filed Mar. 2, 1977, for "PLASTIC COATED STRUCTURE AND METHOD", now U.S. Pat. No. 4,193,180 entitled "Method of Forming a Heat Exchanger", and assigned to the same assignee as the present application, there is described a method for producing a heat exchanger having a series of small diameter tubes radiating from a manifold or header, all of which is coated with a protective layer of plastic. In fabricating the heat exchanger the tubes are jacketed throughout their length with a fluorocarbon resin by extruding the resin directly thereon. After cutting back the jacket for a short distance from an end of the tube, said tube end is joined to the manifold by brazing. Thereafter, the joint, the exposed surfaces of said tube between said joint and the adjacent end of the jacket, a narrow circumferential band of the jacket at said adjacent end of the jacket, and the manifold are coated with the same resin by spraying in dry powder form and then applying heat to fuse the sprayed on resin. It has been found, however, that such process of dry powder coating the manifold and joints is expensive to practice.
As explained in said prior application, the construction of a heat exchanger for operation in a corrosive environment necessitates balancing conflicting design requirements. Optimally, the heat exchanger tubes are constructed of material having maximum heat transmittance with adequate structural strength, minimum wall thickness and resistance to chemical attack by the media to be encountered both within and without. Structural strength requirements are determined by internal and external fluid pressures as well as beam loads due to the deadweight of the tubes and dynamic loads from motion in the form of vibration or acceleration or both. A good compromise is to use a metal tube externally coated or jacketed with a corrosion resistant material. Heat exchanger tubes made entirely of fluorinated ethylene propylene (FEP) resin are known and such resin is suitable as an exterior coating for the foregoing purpose. Also suitable are polyvinylidene fluoride (PVF.sub.2) and perfluoroalkoxy (PFA) resins. The problem sought to be solved by the invention disclosed in said prior application was to obtain a complete encasement of the metal of the heat exchanger, not only that of the tubes but the metal of the associated manifolds or the like, free from pinholes yet sufficiently thin so as not to cause inefficient heat transfer.
With the foregoing in mind, it is an object of the present invention to provide an alternative construction for the heat exchanger structure with has equivalent corrosion resistant integrity but is much less costly to fabricate. In general, said object has been achieved by forming the manifolds of headers from stainless steel or other corrosion resistant material and effecting a suitable corrosion resistant joint between copper tube heat exchange elements and said manifolds.
A further object of the present invention is to achieve a reliable, corrosion resistant junction between the resin coated small-bore heat exchange tubes and the uncoated, inherently corrosion resistant, manifold or header tubes. While not limited thereto, the specific example disclosed in the prior application and disclosed herein relates to a heat exchanger element in which the heat exchange tubes prior to jacketing have an outside diameter of the order of 0.125 inches and an inside diameter of the order of 0.085 inches. As with the prior application, the copper tubes are jacketed by extruding directly thereupon a coating of PFA resin to a thickness of about 5 mils. The manifold or header tubes have, for example, an outside diameter between 3/8ths and 1/2 inches.
In accordance with one aspect of the present invention, there is provided a heat exchanger element for use in a corrosive environment comprising a metal tube jacketed with an extruded layer of a given fluorocarbon resin having a thickness within the limits affording efficient heat transfer combined with resistance to corrosion of said tube, said given resin being of the type which has a high viscosity in the melt stage, a nipple of corrosion resistant metal, said nipple having a first end with a circumferential shoulder at said end and an axial region adjacent said shoulder which region has a lesser diameter than said shoulder providing an undercut annular groove therebehind, said first end of said nipple being telescopingly engaged within an end of said metal tube and having an opposite end which projects from said metal tube, a sleeve of fluorocarbon resin surrounding said end of said metal tube and at least a portion of said opposite end of said nipple, said sleeve extending over said metal tube to a point beyond the location of said shoulder, and a corrosion resistant metal ferrule surrounding said sleeve of resin and substantially coextensive, said ferrule being contracted radially squeezing said metal tube at least within said axial region behind said shoulder thereby effecting interlocking engagement between said metal tube and said nipple and establishing a fluid-tight seal between said ferrule, said nipple and said metal tube.
In accordance with a further aspect of the present invention, there is provided a method for fabricating the structure described above.